Index: llvm/test/Transforms/SimplifyCFG/merge-phis-in-switch.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SimplifyCFG/merge-phis-in-switch.ll
@@ -0,0 +1,383 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+
+; Test a bunch of cases where the combination of phis in switch should be merged into one phi
+
+declare void @use(i8)
+
+define i8 @phis_of_switch_minimal(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch_minimal
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE01:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE01]]
+; CHECK:       case01:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 2, [[CASE1]] ], [ 1, [[START:%.*]] ]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[CASE01]] ], [ 3, [[START]] ]
+; CHECK-NEXT:    ret i8 [[PHI2]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case01
+  i8 1, label %case1
+  i8 2, label %end
+  ]
+unreachable:
+  unreachable
+case1:
+  br label %case01
+
+case01:
+  %phi1 = phi i8 [ 2, %case1 ], [1, %start]
+  br label %end
+
+end:
+  %phi2 = phi i8 [ %phi1, %case01 ], [ 3, %start ]
+  ret i8 %phi2
+}
+
+define i8 @phis_of_switch(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE012:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE2:%.*]]
+; CHECK-NEXT:    i8 3, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case2:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case012:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 3, [[CASE2]] ], [ 2, [[CASE1]] ], [ 1, [[START:%.*]] ]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[CASE012]] ], [ 4, [[START]] ]
+; CHECK-NEXT:    ret i8 [[PHI2]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case012
+  i8 1, label %case1
+  i8 2, label %case2
+  i8 3, label %end
+  ]
+unreachable:
+  unreachable
+case1:
+  br label %case012
+
+case2:
+  br label %case012
+
+case012:
+  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [1, %start]
+  br label %end
+
+end:
+  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
+  ret i8 %phi2
+}
+
+define i8 @multiple_phis_of_switch(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @multiple_phis_of_switch
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE012:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE2:%.*]]
+; CHECK-NEXT:    i8 3, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case2:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case012:
+; CHECK-NEXT:    [[PHI1_1:%.*]] = phi i8 [ 3, [[CASE2]] ], [ 2, [[CASE1]] ], [ 1, [[START:%.*]] ]
+; CHECK-NEXT:    [[PHI1_2:%.*]] = phi i8 [ 6, [[CASE2]] ], [ 5, [[CASE1]] ], [ 4, [[START]] ]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI2_1:%.*]] = phi i8 [ [[PHI1_1]], [[CASE012]] ], [ 4, [[START]] ]
+; CHECK-NEXT:    [[PHI2_2:%.*]] = phi i8 [ [[PHI1_1]], [[CASE012]] ], [ 5, [[START]] ]
+; CHECK-NEXT:    [[PHI2_3:%.*]] = phi i8 [ [[PHI1_2]], [[CASE012]] ], [ 3, [[START]] ]
+; CHECK-NEXT:    call void @use(i8 [[PHI2_1]])
+; CHECK-NEXT:    call void @use(i8 [[PHI2_2]])
+; CHECK-NEXT:    call void @use(i8 [[PHI2_3]])
+; CHECK-NEXT:    ret i8 [[PHI2_1]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case012
+  i8 1, label %case1
+  i8 2, label %case2
+  i8 3, label %end
+  ]
+unreachable:
+  unreachable
+case1:
+  br label %case012
+
+case2:
+  br label %case012
+
+case012:
+  %phi1_1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [1, %start]
+  %phi1_2 = phi i8 [ 6, %case2 ], [ 5, %case1 ], [4, %start]
+  br label %end
+
+end:
+  %phi2_1 = phi i8 [ %phi1_1, %case012 ], [ 4, %start ]
+  %phi2_2 = phi i8 [ %phi1_1, %case012 ], [ 5, %start ]
+  %phi2_3 = phi i8 [ %phi1_2, %case012 ], [ 3, %start ]
+  call void @use(i8 %phi2_1)
+  call void @use(i8 %phi2_2)
+  call void @use(i8 %phi2_3)
+  ret i8 %phi2_1
+}
+
+define i8 @phis_of_switch_multiple_stage0(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch_multiple_stage0
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE0:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE2:%.*]]
+; CHECK-NEXT:    i8 3, label [[CASE0123:%.*]]
+; CHECK-NEXT:    i8 4, label [[CASE01234:%.*]]
+; CHECK-NEXT:    i8 5, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case0:
+; CHECK-NEXT:    br label [[CASE0123]]
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE0123]]
+; CHECK:       case2:
+; CHECK-NEXT:    br label [[CASE0123]]
+; CHECK:       case0123:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 4, [[START:%.*]] ], [ 3, [[CASE2]] ], [ 2, [[CASE1]] ], [ 1, [[CASE0]] ]
+; CHECK-NEXT:    br label [[CASE01234]]
+; CHECK:       case01234:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[CASE0123]] ], [ 5, [[START]] ]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI3:%.*]] = phi i8 [ [[PHI2]], [[CASE01234]] ], [ 6, [[START]] ]
+; CHECK-NEXT:    ret i8 [[PHI3]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case0
+  i8 1, label %case1
+  i8 2, label %case2
+  i8 3, label %case0123
+  i8 4, label %case01234
+  i8 5, label %end
+  ]
+unreachable:
+  unreachable
+
+case0:
+  br label %case0123
+
+case1:
+  br label %case0123
+
+case2:
+  br label %case0123
+
+case0123:
+  %phi1 = phi i8 [4, %start], [ 3, %case2 ], [ 2, %case1 ], [ 1, %case0 ]
+  br label %case01234
+
+case01234:
+  %phi2 = phi i8 [ %phi1, %case0123 ], [ 5, %start ]
+  br label %end
+
+end:
+  %phi3 = phi i8 [ %phi2, %case01234 ], [6, %start]
+  ret i8 %phi3
+}
+
+define i8 @phis_of_switch_multiple_stage1(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch_multiple_stage1
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE0:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE012:%.*]]
+; CHECK-NEXT:    i8 3, label [[CASE3:%.*]]
+; CHECK-NEXT:    i8 4, label [[CASE4:%.*]]
+; CHECK-NEXT:    i8 5, label [[CASE345:%.*]]
+; CHECK-NEXT:    i8 6, label [[CASE0123456:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case0:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case012:
+; CHECK-NEXT:    [[PHI123:%.*]] = phi i8 [ 3, [[START:%.*]] ], [ 2, [[CASE1]] ], [ 1, [[CASE0]] ]
+; CHECK-NEXT:    br label [[CASE0123456]]
+; CHECK:       case3:
+; CHECK-NEXT:    br label [[CASE345]]
+; CHECK:       case4:
+; CHECK-NEXT:    br label [[CASE345]]
+; CHECK:       case345:
+; CHECK-NEXT:    [[PHI456:%.*]] = phi i8 [ 6, [[START]] ], [ 5, [[CASE4]] ], [ 4, [[CASE3]] ]
+; CHECK-NEXT:    br label [[CASE0123456]]
+; CHECK:       case0123456:
+; CHECK-NEXT:    [[PHI1234567:%.*]] = phi i8 [ 7, [[START]] ], [ [[PHI456]], [[CASE345]] ], [ [[PHI123]], [[CASE012]] ]
+; CHECK-NEXT:    ret i8 [[PHI1234567]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case0
+  i8 1, label %case1
+  i8 2, label %case012
+  i8 3, label %case3
+  i8 4, label %case4
+  i8 5, label %case345
+  i8 6, label %case0123456
+  ]
+unreachable:
+  unreachable
+case0:
+  br label %case012
+
+case1:
+  br label %case012
+
+case012:
+  %phi123 = phi i8 [3, %start], [ 2, %case1 ], [ 1, %case0 ]
+  br label %case0123456
+
+case3:
+  br label %case345
+
+case4:
+  br label %case345
+
+case345:
+  %phi456 = phi i8 [6, %start], [ 5, %case4 ], [ 4, %case3 ]
+  br label %case0123456
+
+case0123456:
+  %phi1234567 = phi i8 [7, %start], [ %phi456, %case345 ], [ %phi123, %case012 ]
+  ret i8 %phi1234567
+
+}
+
+define i8 @phis_of_switch_extrause_fail(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch_extrause_fail
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE012:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE2:%.*]]
+; CHECK-NEXT:    i8 3, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case2:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case012:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 3, [[CASE2]] ], [ 2, [[CASE1]] ], [ 1, [[START:%.*]] ]
+; CHECK-NEXT:    call void @use(i8 [[PHI1]])
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[CASE012]] ], [ 4, [[START]] ]
+; CHECK-NEXT:    ret i8 [[PHI2]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case012
+  i8 1, label %case1
+  i8 2, label %case2 i8 3, label %end
+  ]
+unreachable:
+  unreachable
+case1:
+  br label %case012
+
+case2:
+  br label %case012
+
+case012:
+  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [1, %start]
+  call void @use(i8 %phi1)
+  br label %end
+
+end:
+  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
+  ret i8 %phi2
+}
+
+define i8 @phis_of_switch_extra_use_fail(i8 noundef %arg) {
+; CHECK-LABEL: define i8 @phis_of_switch_extra_use_fail
+; CHECK-SAME: (i8 noundef [[ARG:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    switch i8 [[ARG]], label [[UNREACHABLE:%.*]] [
+; CHECK-NEXT:    i8 0, label [[CASE012:%.*]]
+; CHECK-NEXT:    i8 1, label [[CASE1:%.*]]
+; CHECK-NEXT:    i8 2, label [[CASE2:%.*]]
+; CHECK-NEXT:    i8 3, label [[END:%.*]]
+; CHECK-NEXT:    ]
+; CHECK:       unreachable:
+; CHECK-NEXT:    unreachable
+; CHECK:       case1:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case2:
+; CHECK-NEXT:    br label [[CASE012]]
+; CHECK:       case012:
+; CHECK-NEXT:    [[PHI1:%.*]] = phi i8 [ 3, [[CASE2]] ], [ 2, [[CASE1]] ], [ 1, [[START:%.*]] ]
+; CHECK-NEXT:    call void @use(i8 [[PHI1]])
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8 [ [[PHI1]], [[CASE012]] ], [ 4, [[START]] ]
+; CHECK-NEXT:    ret i8 [[PHI2]]
+;
+start:
+  switch i8 %arg, label %unreachable [
+  i8 0, label %case012
+  i8 1, label %case1
+  i8 2, label %case2 i8 3, label %end
+  ]
+unreachable:
+  unreachable
+case1:
+  br label %case012
+
+case2:
+  br label %case012
+
+case012:
+  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [1, %start]
+  call void @use(i8 %phi1)
+  br label %end
+
+end:
+  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
+  ret i8 %phi2
+}